I was quite surprised to learn that Johnson
already used the disc capacitors as shown in an advertisment in
QST oct 52.

This note was
presented in Radcom Technical topics March 1982, but couldn't be
published in QST because they are dependent upon their
advertizers and therefore must adjust the "truth" to
match optimum income for ARRL. My note was therefore denied with
an explanation saying that the two filters I had tested should
possibly have been faulty.

In November 1968 two Collins Radio engineers, wrting in QST
introduced the concept of absorbtive low-pass filters in which
the unwanted high-frequency energy is separated from the wanted
output and then safely dissipated in matched resistors. This
technique was later endorsed by Hans Rohrbacher, DJ2NN in CQ-DL
August 76, and his design appears also in recent editions of ART.
However, it has to be said that the majority of filters continue
to be of orthodox design, in which the transmitter 'sees' a high
or low reactance at high frequencies and which assumes that the
transmitter output impedance at these frequencies is similar to
that of the operating frequency.
In 1981 LA8AK, in conjunction with LA1VC, has been checking
whether in fact absorbtive filtering does offer any substantial
improvement. Some Drake TV-3300-LP filters were modified, using
worst case components (ie ceramic rather than silvered mica
capacitors, and standard value rather than non-inductive low
tolerance values) and carefully checked the attenuation and and
input VSWR (fig 4). It can be argued that the output of a signal
generator alone, unlike a transmitter, will (or should) have the
same output impedance at all frequencies and therefore may tend
to underestimate the advantage of the absorbtive approach.
Nevertheless the results are very convincing. Insertion loss and
passband vswr of the modified filter were found very similar to
the unmodified original filter, and minor differences may be
accounted for by manufacturing tolerance; the input vswr over the
stopband, however, was very different (table). While the apparent
fall-off attenuation of the unmodified filter may appear better ,
in practice this is likely to be lost due to high input vswr.
Although the filters show laboratory attenuation over 80dB, it is
probably that in practice 60dB may prove about the limit since
the transmitter chassis will always contribute to some radiation.
It is considered to be important that the transmitter should not
'see' notch filters connected in parallel with the signal, as in
some earlier absorbtive filter constructions. While it is
difficult to determine all potential 'troublesome' parameters of
an LPF, since different transmitters will be affected
differently, it seems that the most important is the VSWR on the
cable between the filter and transmitter. It is also critical
that this cable should be short, as cable resonnances (which may
short-circuit the output) will introduce problems - and indeed
such techniques are often used to increase harmonic output of the
doublers, triplers etc (idler circuits). In other words, the
absorbtive (or 'hybrid) filter is very desirable beast.

Fig 2a). Insertion loss of original unmodified filter over the
passband 2-30MHz
and between 30-42MHz (marker 33.16MHz) -14.4dBm.
b) Insertion loss of the modified lowpass-filter.

Fig 3a) Stop-band attenuation or the original unmodified filter
b) Stop-band of the modified filter.

Fig 4) Input VSWR of the "worst-case" modified filter
over the range 30-240MHz.

Fig.1 Double-stub notch filters for 144MHz. A and C 17cm, D and F
about 33cm (to be adjusted with twisted wire in the end), B 34cm
(uncritical), E and G (used only to optimize filters for Band III
TV) 12cm. Note that stub dimensions are affected by velocity
factor (0.66) of the cable.